The number of copies of a gene or transcript present in a genome or cell
An important term in genomics with the advent of whole genome sequencing and generally refers to the number of copies of a gene in a genome. In parasites this multi-copy genes are widespread and facilitate convergent evolution (evolution by recombination not associated with Mendelian inheritance). High gene copy number can lead to high levels of gene expression. Copy number transcripts are an alternative definition and can be subject to very difference control mechanisms.